Three-group zoom lens

ABSTRACT

A three-group zoom lens includes, in order from the object side, a first lens group of negative refractive power, and second and third lens groups, each of positive refractive power. The first and second lens groups include negative and positive components and the third lens group is a single lens component. All but one lens component may be a single lens element. When zooming from the wide-angle end to the telephoto end, the first and second lens groups move closer together while the second lens group moves farther from the third lens group. The third lens group remains stationary during zooming but moves for focusing. The second lens group includes a diaphragm on its object side. Aspheric lens surfaces are disclosed. The zoom lens satisfies certain conditions for the focal lengths of the zoom lens and a component of the zoom lens, and for Abbe numbers of two lens elements.

BACKGROUND OF THE INVENTION

[0001] Three-group zoom lenses are known in the prior art and have beenwidely used in attempting to provide compact zoom lenses with goodcorrection of aberrations. Recently, digital cameras and video camerashave become highly popular, and in these cameras, small size, high imagequality including low distortion and good correction of otheraberrations, as well as satisfaction of various conditions connected tothe use of solid state image pickup elements in these cameras, have beendesired. Additionally, in digital cameras and video cameras, autofocushas become an essential focusing mode and ever faster autofocusing isdesired.

[0002] The above considerations have resulted in three-group zoomlenses, rather than two-group zoom lenses, being used in such camerasand inner focus or rear focus lenses being used in such cameras becausethese features enable the zoom lens to weight less and enable the drivenlens components to be the ones close to the camera body, whichfacilitates the drive operation of these cameras. For example, JapaneseLaid-Open Patent Applications 2000-284177, 2001-272602, and 2001-296476disclose three-group zoom lenses that use rear focus and are designedfor quick focusing, small size, and high resolution with good aberrationcorrection.

[0003] In the zoom lenses described in Japanese Laid-Open PatentApplication 2000-284177, the spacing between the second lens group andthe third lens group remains nearly constant over the range of zoomingwhile focusing is performed by moving the third lens group in thedirection of the optical axis. Therefore, the proper spacings betweenthe second lens group and the third lens group for focusing at infinitymust be achievable throughout the range of zooming. Thus, it isdifficult to further shorten the overall length of the lens system usinga retractable lens when the various requirements of the lens barrelregarding the required movements of the lens groups are considered.

[0004] Moreover, the zoom lenses described in all three Japanesepublished applications listed above require a cam mechanism for jointlymoving the second lens group and the third lens group during zooming.Hence there is a restriction on shortening the total length of the lenssystem in the case of a retractable lens barrel. Accordingly, with thezoom lenses of the three Japanese published applications listed above,it is difficult to greatly reduce the total length of the opticalsystems with a retractable lens barrel, which limits the compactness ofcameras using such zoom lenses when the cameras are not being used.

BRIEF SUMMARY OF THE INVENTION

[0005] The present invention relates to a three-group zoom lens that hasa zoom ratio of about three times but has a short overall length of theoptical system and favorable correction of aberrations. In particular,the present invention relates to a three-group zoom lens that may beused in a digital camera or a video camera and that may be used with asolid state image pickup element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

[0007]FIG. 1 shows a cross-sectional view of the zoom lens according toEmbodiment 1;

[0008] FIGS. 2A-2B show cross-sectional views of the zoom lens of FIG. 1at the wide-angle end with the focus set at infinity and at the nearpoint, respectively;

[0009] FIGS. 3A-3B show cross-sectional views of the zoom lens of FIG. 1at the telephoto end with the focus set at infinity and at the nearpoint, respectively;

[0010]FIG. 4 shows a cross-sectional view of the zoom lens according toEmbodiment 1 with the zoom lens retracted;

[0011] FIGS. 5A-5D show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 1 at the wide-angle end;

[0012] FIGS. 5E-5H show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 1 at an intermediate position;

[0013] FIGS. 5I-5L show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 1 at the telephoto end;

[0014] FIGS. 6A-6D show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 2 at the wide-angle end;

[0015] FIGS. 6E-6H show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 2 at an intermediate position; and

[0016] FIGS. 6I-6L show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens accordingto Embodiment 2 at the telephoto end.

DETAILED DESCRIPTION

[0017] A general description of the three-group zoom lens of the presentinvention that pertains to all three embodiments of the invention willfirst be described with reference to FIGS. 1-4 that show Embodiment 1.In FIG. 1, lens elements are referenced by the letter L with a subscriptdenoting their order from the object side of the zoom lens along theoptical axis X, from L₁ to L₆. Similarly, radii of curvature of thesurfaces of various optical elements, including the lens surfaces, arereferenced by the letter R with a subscript denoting their order fromthe object side of the zoom lens, from R₁ to R₁₃. The on-axis surfacespacings along the optical axis X of various optical surfaces arereferenced by the letter D with a subscript denoting their order fromthe object side of the zoom lens, from D₁ to D₁₃. In the same manner,the three lens groups are labeled G₁ through G₃ in order from the objectside of the zoom lens and the lens components belonging to each lensgroup are indicated by lower brackets adjacent the labels G₁-G₃.

[0018] The term “lens group” is defined in terms of “lens elements” and“lens components” as explained herein. The term “lens element” is hereindefined as a single transparent mass of refractive material having twoopposed refracting surfaces, which surfaces are positioned at leastgenerally transversely of the optical axis of the zoom lens. The term“lens component” is herein defined as (a) a single lens element spacedso far from any adjacent lens element that the spacing cannot beneglected in computing the optical image forming properties of the lenselements or (b) two or more lens elements that have their adjacent lenssurfaces either in full overall contact or overall so close togetherthat the spacings between adjacent lens surfaces of the different lenselements are so small that the spacings can be neglected in computingthe optical image forming properties of the two or more lens elements.Thus, some lens elements may also be lens components. Therefore, theterms “lens element” and “lens component” should not be taken asmutually exclusive terms. In fact, the terms may frequently be used todescribe a single lens element in accordance with part (a) above of thedefinition of a “lens component.” The term “lens group” is used hereinto define an assembly of one or more lens components that are fixed orare movable as a single unit.

[0019] As shown in FIG. 1, a diaphragm 2 that controls the amount oflight that passes through the zoom lens is included in the second lensgroup G₂. An image pickup device, such as a CCD (not shown), is arrangedat the image plane 3. A low-pass filter or an infrared cutoff filter 1is arranged between the third lens group G₃ and the image plane 3.

[0020]FIGS. 2A and 2B show cross-sectional views of the zoom lens ofFIG. 1 at the wide-angle end with the focus set at infinity and at thenear point, respectively. FIGS. 3A and 3B show cross-sectional views ofthe zoom lens of FIG. 1 at the telephoto end with the focus set atinfinity and at the near point, respectively. In both Embodiments 1 and2 that follow, the near point of focus is 10 cm on the object side fromthe vertex of the object side lens surface of the first lens element L₁when the zoom lens is at the wide-angle end and 80 cm when the zoom lensis at the telephoto end. FIG. 4 shows a cross-sectional view of the zoomlens of FIG. 1 with the zoom lens retracted.

[0021] In order to improve imaging, lens surfaces may be aspheric lenssurfaces. The lens surfaces that are aspheric satisfy the followingequation:

Z=[(Y ² /R)/{1+(1−K·Y ² /R ²)^(1/2) }]+A ₄ Y ⁴ +A ₆ Y ⁶ +A ₈ Y ⁸ +A ₁₀ Y¹⁰  (Equation A)

[0022] where

[0023] Z is the length (in mm) of a line drawn from a point on theaspheric surface at a distance Y from the optical axis to the tangentialplane of the aspheric surface vertex,

[0024] R is the radius of curvature (in mm) of the aspheric surface onthe optical axis,

[0025] Y is the distance (in mm) from the optical axis,

[0026] K is the eccentricity, and A₄, A₆, A₈, and A₁₀ are the 4th, 6th,8th, and 10th aspheric coefficients.

[0027] As shown in FIG. 1, the three-group zoom lens of the presentinvention includes, in order from the object side, a first lens group G₁having negative refractive power, a second lens group G₂ having positiverefractive power, and a third lens group G₃ having positive refractivepower. The three-group zoom lens is constructed so that the first lensgroup G₁ and the second lens group G₂ are moved closer together and thesecond lens group G₂ is moved farther away from the third lens group G₃during zooming from the wide-angle end to the telephoto end. On theother hand, the third lens group G₃ is stationary during zooming and ismoved toward the object side when focusing the zoom lens from infinityto a near point. The focal length f of the entire zoom lens is changedand a light beam is focused on the imaging plane 3 efficiently by movingthe three lens groups, G₁, G₂, G₃, along the optical axis X in thismanner.

[0028] In the following descriptions, references will be frequently madeto a “lens element.” However, as set forth above, it is understood thatnumerous of the lens elements described below are also lens componentsand may be replaced by lens components that include more than one lenselement.

[0029] The first lens group G₁ includes, in order from the object side,a first lens element L₁ having negative refractive power and a meniscusshape with its concave lens surface on its image side and a second lenselement L₂ having positive refractive power and a meniscus shape withits convex lens surface on its object side. The second lens group G₂includes, in order from the object side, a lens component formed of athird lens element L₃ that is a biconvex lens element and a fourth lenselement L₄ that is a biconcave lens element and that is intimatelybonded to the third lens element L₃, and a fifth lens element L₅ ofmeniscus shape with a concave lens surface on its image side. The term“intimately bonded” is defined herein generally to mean that adjacentrefractive surfaces of two lens elements have substantially the samecurvature and are held in direct fixed contact or are separated by athin layer of transparent adhesive (too thin to be considered in opticalcomputations) that fixes the lenses together, the latter being commonlyreferred to as a “cemented” lens element arrangement.

[0030] Additionally, in both Embodiments 1 and 2 to be more fullydescribed below, the second lens group G₂ includes a diaphragm 2 on theobject side of the third lens element L₃ that controls the amount oflight that passes through the zoom lens.

[0031] The third lens group G₃ includes a sixth lens element L₆ that isa biconvex lens element. On the image side of the sixth lens element L₆is the filter 1 followed by the image plane 3. The third lens group G₃is set in a reference retracted position corresponding to the focusbeing at infinity and in a lens barrel situated, for example, in acamera case. The third lens group G₃ is driven independently of theother lens groups G₁ and G₂ for focusing by an actuator (not shown, suchas a motor) so as to move along the optical axis X. Thus, the third lensgroup G₃ can be moved quickly and easily to the retracted position shownin FIG. 4, providing a compact arrangement of lens groups, therebyproviding a very short zoom lens in the retracted position.

[0032] As shown in FIGS. 1-4, lens elements L₄ and L₅ include planarperipheral portions parallel to one another and perpendicular to theoptical axis X that are in direct contact with one another or separatedby a plane parallel plate. In general, the use of such planar parallelportions improves the assembly accuracy of the lens components of thesecond lens group G₂, including reducing problems of decentering of thelens elements along the optical axis X.

[0033] Preferably, the three-group zoom lens satisfies the followingcondition:

f _(w) /|f _(2-f)|<0.2  Condition (1)

[0034] where

[0035] f_(w) is the focal length of the entire three-group zoom lens atthe wide-angle end, and

[0036] f_(2-f) is the focal length of the image-side lens component ofthe second lens group G₂, which as shown in FIGS. 1-4 is the fifth lenselement L₅.

[0037] When Condition (1) above is satisfied, the focus shift due tochanges in the ambient temperature and the ambient humidity are reduced.Additionally, Condition (1) may be satisfied by the meniscus lenselement L₅ being made of plastic which allows the lens element to bemade inexpensively. Additionally, the meniscus lens element L₅ can bemade to include aspheric lens surfaces easily and inexpensively when itis made of plastic.

[0038] Additionally, preferably the three-group zoom lens satisfies thefollowing condition:

ν₃−ν₄>14  Condition (2)

[0039] where

[0040] ν₃ is the Abbe number of the biconvex lens element of the secondlens group G₂, and

[0041] ν₄ is the Abbe number of the biconcave lens element of the secondlens group G₂.

[0042] Condition (2) limits the difference in Abbe numbers of thematerials of the two lens elements of the second lens group G₂ that areintimately bonded together. By satisfying Condition (2), the lateralcolor at the wide-angle end and the axial chromatic aberration at thetelephoto end can be well corrected.

[0043] By making the refractive power of the lens element L₅ in thesecond lens group negative, the composite refractive power of this lensand the positive lens L₆ that forms the third lens group is decreased.If both the lenses L₅ and L₆ are made of plastic, production costs canbe reduced and the overall image quality enhanced. Thus, making therefractive power of the lens element L₅ negative decreases the focusshift due to temperature/humidity changes. Because the compositerefractive power of the lenses L₅ and L₆ is less, negative distortiongenerated at the wide-angle end can be suppressed, and the amount ofdistortion correction required at the aspheric lens surfaces of thefirst lens element L₁ can be reduced. At the same time, the correctionof field curvature at the wide-angle end can be improved so as toimprove the overall image quality.

[0044] Because in the three-group zoom lens of the present invention thethird lens group G₃ is fixed at a predetermined reference positioncorresponding to a focus position at infinity when the zoom lens isretracted, from which it, and it alone, moves during focusing, thefocusing operation is easily achieved by an actuator, such as a motor,and the retracted length of the entire zoom lens may be made very short.Additionally, because lens elements that are intimately bonded togetherare used in the second lens group G₂, there is no air space betweenthese lens elements, which results in zero spacing between them so thatthe second lens group G₂ can be made thinner, assisting in making thelength of the entire zoom lens ever shorter. Furthermore, such aconstruction, along with the first lens group G₁ and the second lensgroup G₂ including at least one aspheric lens surface, enables thethree-group zoom lens to achieve favorable correction of aberrationseven if only a total of six lens elements are used in the three-groupzoom lens.

[0045] Embodiments 1 and 2 of the present invention will now beindividually described with further reference to the drawings.

Embodiment 1

[0046] In Embodiment 1, as shown in FIGS. 1-4, the fifth lens element L₅has negative refractive power. Additionally, the sixth lens element L₆has a lens surface of greater curvature on the image side and both lenssurfaces of the first lens element L, and the fifth lens element L₅ areaspheric lens surfaces.

[0047] Table 1 below lists the surface number #, in order from theobject side, the radius of curvature R (in mm) of each surface near theoptical axis, the on-axis surface spacing D (in mm) between surfaces, aswell as the refractive index N_(d) and the Abbe number ν_(d) (at thed-line of 587.6 nm) of each lens element for Embodiment 1. Listed in thebottom portion of Table 1 are the focal length f and the f-number F_(NO)at the wide-angle and telephoto ends, and the maximum image angle 2ω atthe wide-angle end and telephoto end for Embodiment 1. TABLE 1 # R DN_(d) ν_(d)  1* 55.1184 1.00 1.80348 40.4  2* 4.1000 1.40  3 7.0428 2.191.84666 23.8  4 19.3593 D₄ (variable)  5 (stop) ∞ 0.50  6 5.4370 2.991.77250 49.6  7 −5.4370 0.56 1.72151 29.2  8 9.8887 0.45  9* 5.7325 1.001.50869 56.0 10* 5.3643 D₁₀ (variable) 11 20.8289 1.70 1.50869 56.0 12−18.5391 2.98 13 ∞ 1.30 1.51680 64.2 f = 5.2-14.40 mm F_(NO) = 2.9-5.02ω = 59.6° − 22.4°

[0048] The lens surfaces with a * to the right of the surface number inTable 1 are aspheric lens surfaces, and the aspheric surface shape ofthese lens elements is expressed by Equation (A) above.

[0049] Table 2 below lists the values of the constants K, A₄, A₆, A₈,and A₁₀ used in Equation (A) above for each of the aspheric lenssurfaces of Table 1. Aspheric coefficients that are not present in Table2 are zero. An “E” in the data indicates that the number following the“E” is the exponent to the base 10. For example, “1.0E−2” represents thenumber 1.0×10⁻². TABLE 2 # K A₄ A₆ A₈ A₁₀ 1 1.282695 −0.2826525E−30.1971951E−4 −0.5887686E−6 0.7724510E−8 2 −0.780090 0.1803997E−2−0.4271172E−5 0.6618535E−6 −0.1738877E−7 9 2.510877 −0.1615613E−2−0.3162416E−3 −0.3309882E−4 −0.1462002E−6 10 −2.834706 0.6604927E−2−0.2519868E−3 −0.2436108E−4 −0.1082942E−6

[0050] In the zoom lens of Embodiment 1, lens groups G₁ and G₂ move tovary their separations and the separation of lens group G₂ from lensgroup G₃ during zooming. Therefore, the values of the on-axis spacingsD₄ and D₁₀ vary. Table 3 below lists the values of the variables D₄ andD₁₀ (i.e., the on-axis spacings) at the wide-angle end (Wide), at anintermediate position, and at the telephoto end (Tele). The focal lengthof the zoom lens when focused at infinity is 5.2 mm at the wide-angleend, 8.63 mm at the intermediate position, and 14.4 mm at the telephotoend. TABLE 3 # Wide Intermediate Tele D₄ 13.11 6.44 2.39 D₁₀ 4.82 8.5214.74

[0051] The zoom lens of Embodiment 1 of the present invention satisfiesboth Conditions (1) and (2) above as set forth in Table 4 below. TABLE 4Condition No. Condition Value (1) f_(w)/|f_(2−f)| < 0.2 0.0026 (2) ν₃ −ν₄ > 14 20.4

[0052] FIGS. 5A-5D show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens ofEmbodiment 1 at the wide-angle end. FIGS. 5E-5H show the sphericalaberration, astigmatism, distortion, and lateral color, respectively, ofthe zoom lens of Embodiment 1 at the intermediate position, and FIGS.5I-5L show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom lens of Embodiment 1 at thetelephoto end. In FIGS. 5A, 5E, and 5I, the spherical aberration isshown for the wavelengths 587.6 nm (the d-line), 460.0 nm, and 615.0 nm.In the remaining figures, ω is the half-image angle. In FIGS. 5B, 5F and5J, the astigmatism is shown for the sagittal image surface by solidline curves and the tangential image surface by dotted line curves. InFIGS. 5C, 5G and 5K, distortion is measured at 587.6 nm (the d-line). InFIGS. 5D, 5H and 5L, the lateral color is shown for the wavelengths460.0 nm and 615.0 nm relative to 587.6 nm (the d-line). As is apparentfrom these figures, the various aberrations are favorably corrected overthe entire zoom range.

Embodiment 2

[0053] Embodiment 2 is very similar to Embodiment 1 and therefore onlythe differences between Embodiment 2 and Embodiment 1 will be explained.Embodiment 2 differs from Embodiment 1 in its lens element configurationonly by different radii of curvature of lens surfaces, differenteccentricities and aspheric coefficients of the aspheric lens surfaces,different optical element surface spacings, and some differentrefractive indexes and Abbe numbers of the materials of lens elements.Because of these differences, in Embodiment 2, the fifth lens element L₅has positive refractive power, rather than negative refractive power asin Embodiment 1, and the sixth lens element L₆ has a lens surface ofgreater curvature on the object side, rather than on the image side asin Embodiment 1.

[0054] Table 5 below lists the surface number #, in order from theobject side, the radius of curvature R (in mm) of each surface near theoptical axis, the on-axis surface spacing D (in mm) between surfaces, aswell as the refractive index N_(d) and the Abbe number ν_(d) (at thed-line of 587.6 nm) of each lens element for Embodiment 2. Listed in thebottom portion of Table 5 are the focal length f and the f-number F_(NO)at the wide-angle and telephoto ends, and the maximum image angle 2ω atthe wide-angle end and the telephoto end for Embodiment 2. TABLE 5 # R DN_(d) ν_(d)  1* 24.521 1.00 1.80348 40.4  2* 4.5009 2.24  3 8.2926 1.911.92286 20.9  4 14.3578 D₄ (variable)  5 (stop) ∞ 0.40  6 5.4504 3.381.69680 55.5  7 −5.4504 0.56 1.64769 33.8  8 7.3050 0.20  9* 4.9983 0.951.50869 56.0 10* 6.1706 D₁₀ (variable) 11 17.4663 1.87 1.48749 70.2 12−31.7154 3.43 13 ∞ 1.05 1.51680 64.2 f = 5.84-16.05 mm F_(NO) = 2.9-5.02ω = 62.2° − 23.8°

[0055] The lens surfaces with a * to the right of the surface number inTable 5 are aspheric lens surfaces, and the aspheric surface shape ofthese lens elements is expressed by Equation (A) above.

[0056] Table 6 below lists the values of the constants K, A₄, A₆, A₈,and A₁₀ used in Equation (A) above for each of the aspheric lenssurfaces of Table 5. Aspheric coefficients that are not present in Table6 are zero. An “E” in the data indicates that the number following the“E” is the exponent to the base 10. For example, “1.0E−2” represents thenumber 1.0×10⁻². TABLE 6 # K A₄ A₆ A₈ A₁₀ 1 −1.897722 −0.4267753E−30.1584805E−4 −0.2595439E−6 0.1597127E−8 2 −0.302062 0.4537918E−30.1219269E−4 0.4610195E−6 −0.1312182E−7 9 0.141779 0.9503975E−30.4713450E−4 −0.6622317E−4 0.2438356E−5 10 −6.084744 0.7852092E−2−0.1797004E−3 −0.2113089E−4 −0.1514149E−5

[0057] In the zoom lens of Embodiment 2, lens groups G₁ and G₂ move tovary their separations and the separation of lens group G₂ from lensgroup G₃ during zooming. Therefore, the values of the on-axis spacingsD₄ and D₁₀ vary. Table 7 below lists the values of the variables D₄ andD₁₀ (i.e., the on-axis spacings) at the wide-angle end (Wide), at theintermediate position, and at the telephoto end (Tele). The focal lengthof the zoom lens when focused at infinity is 5.84 mm at the wide-angleend, 9.69 mm at the intermediate position, and 16.05 mm at the telephotoend. TABLE 7 # Wide Intermediate Tele D₄ 14.20 7.11 2.85 D₁₀ 6.12 10.3717.40

[0058] The zoom lens of Embodiment 2 of the present invention satisfiesboth Conditions (1) and (2) above as set forth in Table 8 below. TABLE 8Condition No. Condition Value (1) f_(w)/|f_(2−f)| < 0.2 0.144 (2) ν₃ −ν₄ > 14 21.7

[0059] FIGS. 6A-6D show the spherical aberration, astigmatism,distortion, and lateral color, respectively, of the zoom lens ofEmbodiment 2 at the wide-angle end. FIGS. 6E-6H show the sphericalaberration, astigmatism, distortion, and lateral color, respectively, ofthe zoom lens of Embodiment 2 at the intermediate position, and FIGS.6I-6L show the spherical aberration, astigmatism, distortion, andlateral color, respectively, of the zoom lens of Embodiment 2 at thetelephoto end. In FIGS. 6A, 6E, and 6I, the spherical aberration isshown for the wavelengths 587.6 nm (the d-line), 460.0 nm, and 615.0 nm.In the remaining figures, ω is the half-image angle. In FIGS. 6B, 6F and6J, the astigmatism is shown for the sagittal image surface by solidline curves and the tangential image surface by dotted line curves. InFIGS. 6C, 6G and 6K, distortion is measured at 587.6 nm (the d-line). InFIGS. 6D, 6H and 6L, the lateral color is shown for the wavelengths460.0 nm and 615.0 nm relative to 587.6 nm (the d-line). As is apparentfrom these figures, the various aberrations are favorably corrected overthe entire zoom range.

[0060] The present invention is not limited to the aforementionedembodiments, as it will be obvious that various alternativeimplementations are possible. For example, the number of lens elementsand lens components, values such as the radius of curvature R of each ofthe lens elements and components, the surface spacings D, the refractiveindex N_(d), as well as the Abbe number ν_(d), are not limited to theexamples indicated in each of the aforementioned embodiments, as othervalues can be adopted. Such variations are not to be regarded as adeparture from the spirit and scope of the invention. Rather, the scopeof the invention shall be defined as set forth in the following claimsand their legal equivalents. All such modifications as would be obviousto one skilled in the art are intended to be included within the scopeof the following claims.

What is claimed is:
 1. A zoom lens formed of only three lens groups, inorder from the object side, as follows: a first lens group havingnegative refractive power; a second lens group having positiverefractive power; and a third lens group having positive refractivepower; wherein the first lens group includes, in order from the objectside, a negative lens component and a positive lens component; thesecond lens group includes, in order from the object side, a biconvexlens component and a biconcave lens component that is intimately bondedto said biconvex lens component, a meniscus lens component with itsconvex lens surface on its object side, and the second lens groupfurther includes a diaphragm for controlling the amount of light thatpasses through the zoom lens; the third lens group is stationary duringzooming and includes a lens component having positive refractive power,and the third lens group moves toward the object side from a referenceposition during focusing from infinity to a near point; at least onelens component of each of the first and second lens groups includes alens surface of aspheric shape; the first and the second lens groups aremoved so that the first and second lens components become closertogether and so that the second and third lens components become fartherapart during zooming from the wide-angle end to the telephoto end; andthe following condition is satisfied: f _(w) |f _(2-f)|<0.2 where f_(w)is the focal length of the zoom lens at the wide-angle end, and f_(2-f)is the focal length of the image-side lens component of the second lensgroup.
 2. The zoom lens of claim 1, wherein said biconvex lens componentincludes a biconvex lens element, said biconcave lens component includesa biconcave lens element, and the following condition is satisfied:ν₃−ν₄>14 where ν₃ is the Abbe number of said biconvex lens element, andν₄ is the Abbe number of said biconcave lens element.
 3. The zoom lensof claim 1, wherein the second lens group consists of three lenselements.
 4. The zoom lens of claim 1, wherein the third lens groupconsists of a single lens element.
 5. The zoom lens of claim 3, whereinthe third lens group consists of a single lens element.
 6. The zoom lensof claim 1, wherein the zoom lens consists of five lens components. 7.The zoom lens of claim 1, wherein the zoom lens consists of six lenselements.
 8. The zoom lens of claim 1, wherein said reference positionof said third lens group is the position of said third lens group whenthe zoom lens is retracted.
 9. The zoom lens of claim 2, wherein saidreference position of said third lens group is the position of saidthird lens group when the zoom lens is retracted.
 10. The zoom lens ofclaim 1, wherein said meniscus lens component has negative refractivepower.
 11. The zoom lens of claim 2, wherein said meniscus lenscomponent has negative refractive power.
 12. The zoom lens of claim 8,wherein said meniscus lens component has negative refractive power. 13.The zoom lens of claim 1, wherein said biconcave lens component and saidmeniscus lens component include planar peripheral portions parallel toone another and perpendicular to the optical axis of the zoom lens thatare in contact with each other or are separated by a plane parallelplate.
 14. The zoom lens of claim 2, wherein said biconcave lenscomponent and said meniscus lens component include planar peripheralportions parallel to one another and perpendicular to the optical axisof the zoom lens that are in contact with each other or are separated bya plane parallel plate.
 15. The zoom lens of claim 1, wherein saidmeniscus lens component includes a lens surface of aspheric shape. 16.The zoom lens of claim 2, wherein said meniscus lens component includesa lens surface of aspheric shape.
 17. The zoom lens of claim 8, whereinsaid meniscus lens component includes a lens surface of aspheric shape.18. The zoom lens of claim 1, wherein said meniscus lens component ismade of plastic.
 19. The zoom lens of claim 2, wherein said meniscuslens component is made of plastic.
 20. The zoom lens of claim 8, whereinsaid meniscus lens component is made of plastic.